1. Field of the Invention
The present invention relates to a fault diagnosis apparatus for diagnosing a vehicle passenger protecting device such as an air bag mounted on the vehicle.
2. Description of the Prior Art
FIG. 1 is a block diagram of a prior art control circuit of a vehicle passenger protecting device as disclosed in Japanese Patent Laid-Open No. HEI/3-238359. This control circuit controls the ignition of a squib 6 that actuates illustratively an air bag, not shown. The squib 6 is an example of the means for activating the vehicle passenger protecting device such as the air bag. In FIG. 1, reference numeral 20 represents collision detecting means comprising a first acceleration sensor 1, a second acceleration sensor 2 and a collision judging circuit 3. The first and the second acceleration sensors 1 and 2 detect the acceleration of the vehicle, and the collision judging circuit 3, implemented illustratively as a microcomputer, checks to see if the vehicle has collided. Reference numeral 4 stands for a NAND circuit that outputs the inverted logical product of an output a from the collision judging circuit 3 and the output from a delay circuit 9. Reference numeral 7 denotes an AND circuit that outputs the logical product of an output b from the collision judging circuit 3 and the output from the delay circuit 9.
Reference numeral 5 is a PNP transistor whose base is connected to the output of the NAND circuit 4; 8 is an NPN transistor 8 whose base is connected to the output of the AND circuit 7. A power supply 11, the PNP transistor 5, the squib 6, the NPN transistor 8 and ground are connected in cascade connection. A capacitor, not shown, for accumulating electrical energy is furnished interposingly between the power supply 11 and the PNP transistor 5.
In operation, the first and the second acceleration sensors 1 and 2 detect the acceleration of the vehicle and supply the collision judging circuit 3 thereof. The collision judging circuit 3 integrates the acceleration from the first acceleration sensor 1 and regards the result as a first: integral value. If the first integral value exceeds a first threshold value, the collision judging circuit 3 brings its output a to the high level. The collision judging circuit 3 also integrates the acceleration from the second acceleration sensor 2 and regards the result as a second integral value. If the second integral value exceeds a second threshold value, the output b is brought to the high level. If the difference between the first integral value and the first threshold value is within a predetermined range and if the difference between the second integral value and the second threshold value is also within a predetermined range, the collision judging circuit 3 brings its output c to the high level. The first and the second threshold values correspond to the integral values at which the air bag must be inflated. The predetermined ranges are those that are free from the effects of noises.
The output c is delayed for a certain period of time by the delay circuit 9. When the output a and the output from the delay circuit 9 are at the high level, the NAND circuit 4 brings its output to the low level to turn on the PNP transistor 5. When the output b and the output from the delay circuit 9 are at the high level, the AND circuit 7 brings its output to the high level to turn on the NPN transistor 8. This causes the capacitor to supply its energy to the squib 1 for ignition.
The role of the delay circuit 9 is to prevent malfunction in case of a runaway of the collision judging circuit 3. That is, the operation of the collision judging circuit 3 is monitored at intervals each shorter than the delay time of the delay circuit 9. If any operational fault is detected, the collision judging circuit 3 is reset. As a result, the outputs a through c are brought to the low level to keep the squib 1 from igniting. The monitoring circuit is not included in FIG. 1.
High levels of reliability are required of the control circuit for the air bag or other vehicle passenger protecting devices.. Besides the monitoring by the collision judging circuit 3, the attempts to meet the strict reliability requirement include various measures including one disclosed in Japanese Patent Laid-Open No. HEI/3-16854. The disclosure involves allowing small currents to flow through a transistor arrangement while the squib 1 is being monitored for the voltage it develops. When the voltage exceeds a predetermined range, a fault is considered to have occurred. The setup comprises a current flow control portion and a voltage detecting circuit. The current flow control portion feeds the transistor arrangement with small currents at predetermined intervals.
One disadvantage of the above prior art is that the current flow control portion and the voltage detecting circuit needed for fault diagnosis tend to complicate the circuit structure. The fact that the current flow control portion further requires a current limiting circuit makes the circuit structure even more complicated. In addition, there is a possibility that the air bag can be inflated inadvertently despite the limits applied to the current flowing through the transistors 5 and 8.